1. Field of the Invention
The present invention relates to a vehicle body frame for a saddle-ride vehicle, which may be a motorcycle, and to a motorcycle including the vehicle body frame.
2. Description of the Background Art
In many cases, body frames for motorcycles and other saddle-ride vehicles are manufactured by welding together pipe materials having tubular frames.
Among the known welding methods, arc welding has mainly been used. In recent years, however, laser welding has become common, and has been employed for manufacturing some motorcycle body frames (see, for example, FIGS. 5 and 7 of Patent Document 1).
FIG. 5(A) of Patent Document 1 shows part of a vehicle body frame, which is formed by arranging pipe materials (P1 to P4) (the numbers in parenthesis represent the reference numerals from the cited document; the same applies below) facing towards each other, and then laser-welding their facing portions.
For example, each pipe material (P2) is laser-welded to a pipe material (P1) with a predetermined gap (clearance) therebetween. The end of the pipe material (P1) and the end of the pipe material (P2) are melted by the heat of the laser beam, and then solidified and joined to each other. In this solidification, the molten metal shrinks. Thus, though slightly, the longitudinal dimensions (the lengths in the vehicle length direction) of the pipe material (P1) and the pipe material (P2) shrink. The degree of this shrinkage differs depending on the conditions, thereby causing great variation. Since there are three welding spots (on each side), the longitudinal dimension varies to an unacceptably large extent.
The consequence is that the distance between each reinforcing member (q) and a head pipe shown in FIG. 7 of Patent Document 1 differs from one vehicle body frame to another. A structure is desired which is capable of suppressing the change in the length of a vehicle body frame in the vehicle length direction that occurs during the welding thereof.
Further, in the case of a two- or three-wheeled vehicle, large force (reactive force from a road surface or steering force) is applied to its head pipe through its steering shaft. The head pipe is what is called a strength member and is required to be sufficiently strong.
Since, as noted, they are required to be strong, head pipes have heretofore only rarely been manufactured as a divided structure. In the manufacturing of a one-piece head pipe by casting or forging, it is difficult to form a hollow part in which to insert a steering shaft, and the head pipe is likely to be thick as a whole. Thus, it has been difficult to reduce the weight of the head pipe.
One may conceive of a structure in which a head pipe is formed as a divided structure, and the divided halves are joined together with bolts. In such case, however, it is necessary to extend flanges from the divided halves and also to join the flanges together with bolts. This increases the weight.
To satisfy demands for dimensional accuracy improvement and weight reduction, a vehicle body frame has been desired which has good dimensional accuracy in the vehicle length direction and includes a head pipe which is relatively light in weight.                [Patent Document 1] Japanese Patent Application Publication No. 2000-6869        